1. Field of the Invention
The present invention relates to apparatus and methods for switching an input beam of light between two nonadjacent output positions, such that all of the light appears at the output positions before, during and after switching, and no light appears between the positions. In particular, the present invention is a switch useful for tuning, re-configuring, or switching Optical Add/Drop Multiplexers used with Wavelength Division Multiplexed (WDM) optical fiber communication, such that the through signals (those wavelengths neither being dropped or added) are not affected by the tuning, switching, or re-configuration.
2. Description of the Prior Art
In Wavelength Division Multiplexed (WDM) fiber optics communications, one fiber carries many data streams, each on a separate wavelength signal. In networks using WDM, ideally each node should be able to separate out (drop) any wavelength in use on the fiber and re-direct it to a detector or sub-network. At the same time, it is desirable that each node should be able to add data to the fiber on any wavelength channel that is currently unused at the node, either because such wavelength is not present at the node, or because it was just dropped at said node.
In addition, if network nodes are able to switch between dropping a given wavelength channel and not-dropping it fast enough (in a way that does not interrupt other network traffic while switching), then the network controller can time-share a wavelength to several subscribers. This is highly desirable, since many customers do not want or need the full data rate possible on a single wavelength. A fast enough switching time for this application is probably <2 milliseconds. This highly-desirable mode of operation is enabled by several of the techniques taught by this invention.
In today's optical WDM optical networks, nodes are actually implemented in two different ways, neither of which is ideal:
1. Optical to Electronic to Optical (OEO) Conversion: This is the most common (and expensive) method of constructing nodes. All wavelengths coming into the node along the input fiber are demultiplexed into separate channels and detected (i.e., converted to electronic signals). The signals which are not being dropped at the node are used to modulate lasers and the resulting wavelengths are multiplexed back onto the output fiber. The multiplexing/demultiplexing is typically done with either arrays of filters or with diffraction grating techniques.
The advantage of this method is that the node is completely flexible—any wavelength can be dropped or added at the node. In addition, signals may be transferred from one wavelength to another.
The disadvantages of this method are:
a) Expensive hardware components (the detector, electronics, laser, and modulator) are needed for each wavelength on the fiber. This rapidly becomes very expensive as numbers of wavelengths grow.
b) Much of the hardware (detectors, electronics, and laser modulators) are data-rate dependent If the network is upgraded from 2.5 Gigabits/sec to 10 Gigabits/sec per wavelength, for example, all electronics at all nodes must also be expensively upgraded.
2. Fixed Optical Add/Drop Filters: There are, at most, two nodes in a WDM network (the terminal nodes) that need to drop all wavelengths on the fiber—all other nodes (intermediate nodes) usually need to drop or add only a few wavelengths. This can be done inexpensively by passing the fiber through several fixed-wavelength drop/add filters. Only the wavelengths these filters are designed for are dropped or added—all other wavelengths simply continue on with no change. These filters are usually constructed using thin film interference filters or fiber Bragg gratings.
Advantages: This node style is considerable less expensive than an OEO node—filters, electronics, and lasers are only required for the number of wavelengths actually to be dropped at the node. If the wavelengths are being sent on to a sub-network, only the inexpensive filters are needed, and the node is data-rate independent.
Disadvantages: Fixed-wavelength nodes don't allow the network to adjust to varying loads, and make network expansion more difficult. When the network grows complicated enough, “wavelength blocking” occurs: even though the network may be far from it's theoretical carrying capacity, certain pathways are blocked from use as no single wavelength can connect them. The network could be manually re-configured to remove any given block, but this would create other blocked paths during different load conditions. This problem grows rapidly with network complexity. In addition, current fixed-drop technologies cannot be switched on and off without interrupting the rest of the network traffic.
Neither of the above methods of constructing optical add/drop network nodes adequately address the need for networks to be both inexpensive and easily and quickly reconfigurable: the OEO nodes achieve wavelength flexibility at the cost of a very high price and data-rate sensitivity; the fixed wavelength add/drop filter nodes are data-rate insensitive and inexpensive, but are completely inflexible as to the dropped wavelengths. The ideal network node would, therefore, have the following characteristics:
1. The node would be all optical—there would be no optical to electronic conversions. Thus the node would be completely insensitive to data-rate upgrades.
2. The node would have the flexibility to drop (and add) any wavelength on the fiber, and the wavelengths to drop could be changed remotely at any time without data interruption to the rest of the network.
3. The node could be constructed relatively inexpensively, using proven components.
4. The node would have low loss, at least for the passed (undropped) wavelengths, so as to minimize the requirement for expensive optical amplifiers.
Tunable and Reconfigurable Add/Drop Filters
An ideal method of addressing the above problem would be to have a tunable add/drop filter, where any desired wavelength could be dropped and/or added at a given node, simply by tuning the filter to the desired wavelength. A major problem with most proposals for tunable add/drop filters is that they interrupt wavelengths while tuning. That is, if a tunable drop filter is currently dropping channel 3 (for example), and is commanded to tune to channel 12; channels 4 through 11 are momentarily interrupted (“hit”) as the filter tunes by. Since optical data rates can be as high as 10 Gigabytes/second on each wavelength, significant data is lost to even a millisecond interruption.
A need remains in the art for improved hitless methods of tuning add drop filters. In particular, a need remains in the art for a light switch that allows hitless tuning of the filter by smoothly switching to reflecting all wavelengths while the filter is tuned.